Gopher tortoises (Gopherus polyphemus) are native to the southeastern United States - indeed they are the state reptile of Georgia and the state tortoise of Florida! They are seen as a keystone species (i.e.., very important for the ecosystem) as they dig burrows that provide shelter for many other species. Gopher tortoises are threatened by habitat destruction, mostly from human development (conversion of their habitat to urban areas and farmland, and adverse forest management), and are listed as threatened in several states.
This species of tortoise lives in sandy habitat and inhabits burrows that it digs itself. This makes it hard for observers to detect them during, for example, a visual line transect survey (see also Distance Sampling) where an observer walks down pre-defined transect lines recording the tortoises they detect. While the observer is allowed to miss tortoises further away from the line, conventional distance sampling methods rely on that the observer detects every tortoise that is directly on the line. Hence, missing the tortoises on the line will create bias in the abundance estimate (which is an estimate of how many tortoises are in the study area). We at CREEM have developped methods for estimating this type of bias and adjusting the abundance estimate accordingly, one method being Mark-recapture distance sampling methods.
Some of the key questions in conservation of rare species are 'how many of them are there?' as well as understanding their habitat preferences and movement patterns. In this study, our collaborators in Spain used line transect methods (a type of Distance Sampling) to monitor the endangered spur-thighed tortoises (Testudo graeca) from the southeastern Iberian Peninsula in their natural habitat. Using an advanced type of distance sampling called density surface modelling we estimated their abundances and identified the type of habitat these tortoises prefer.
The endangered spur-thighed tortoises are monitored in the southeastern Iberian Peninsula. Photo: Roberto Rodríguez Caro.
Rodríguez-Caro RC, CS Oedekoven, E Graciá, JD Anadón, ST Buckland, MA Esteve-Selma,J Martinez, A Giménez. 2017. Low tortoise abundances in pine forest plantations in forest-shrubland transition areas. PLOS ONE 12(3): e0173485. doi: 10.1371/journal.pone.0173485